#Glossary #SignalFormats/SDI > [!Question] What is the difference between the 3G-SDI format outputs of SMPTE Level A, and SMPTE Level B? The difference between 3G-SDI format outputs of SMPTE Level A and SMPTE Level B lies in how they handle the data payload and map the video signals onto the SDI interface. These differences affect compatibility and how devices interpret the video signal. ### 3G-SDI Overview 3G-SDI (3 Gigabit Serial Digital Interface) is a standard defined by the Society of Motion Picture and Television Engineers (SMPTE) for transmitting high-definition video over a single coaxial cable. It supports data rates of up to 2.97 Gbps, enabling 1080p video at 60 frames per second. ### SMPTE 425M SMPTE 425M defines two mapping methods for transmitting 1080p video signals over a single 3G-SDI link: Level A and Level B. ### SMPTE Level A - **Signal Mapping**: Level A maps a single 1080p60 video stream directly onto the 3G-SDI link. - **Payload**: It carries one 1080p60 video stream as a single payload. - **Compatibility**: Generally simpler and more straightforward, making it more widely compatible with devices that support 3G-SDI. - **Usage**: Common in broadcast equipment and professional video production environments where straightforward signal routing is preferred. ### SMPTE Level B - **Signal Mapping**: Level B has two sub-levels: Level B-DL (Dual Link) and Level B-Dual Stream. - **Level B-DL**: Maps two 1080i (interlaced) or 720p signals onto a single 3G-SDI link by multiplexing them. Effectively, it combines two separate HD-SDI (1.5 Gbps each) signals into one 3G-SDI stream. - **Level B-Dual Stream**: Supports two independent 1080p30 video streams. - **Payload**: It can carry two 1080i or 720p video streams, or two 1080p30 streams. - **Compatibility**: Requires devices that can correctly interpret the multiplexed signal. Not all 3G-SDI equipment supports Level B. - **Usage**: Used in applications where multiple video streams need to be carried over a single cable, such as in complex broadcast setups or for specific transmission needs. ### Key Differences 1. **Mapping Method**: - **Level A**: Single 1080p60 signal on the link. - **Level B**: Multiplexed signals—either two 1080i/720p signals (Level B-DL) or two 1080p30 signals (Level B-Dual Stream). 2. **Compatibility**: - **Level A**: More universally compatible due to its simpler, direct mapping. - **Level B**: Requires specific support from receiving equipment to demultiplex the signal correctly. 3. **Use Cases**: - **Level A**: Preferred for simplicity and wide compatibility in typical 1080p60 applications. - **Level B**: Beneficial in situations needing multiple streams over one cable, such as redundancy or special broadcast setups. ### Practical Implications When choosing between Level A and Level B for 3G-SDI: - Ensure that all equipment in your signal chain (cameras, monitors, switchers, etc.) supports the chosen level. - Level A is generally easier to implement and troubleshoot due to its straightforward signal structure. - Level B can be advantageous for specific use cases but requires compatible hardware and careful configuration. Understanding these differences will help in setting up and maintaining a robust 3G-SDI infrastructure tailored to your specific video production needs. %% – ChatGPT [[240522Wed]]